Radio receivers are well known in the art. Such receivers typically include an antenna for transducing airborne radio frequency signals into electrical radio frequency signals. A preselector unit selects a predetermined band of the frequency spectrum, and a decoding unit functions to extract information that has been modulated in a preselected fashion with the signal of interest. Radios that can select a carrier signal in the land mobile low-band (30 to 50 MHz), mid-band (66 to 88 MHz), high-band (132 to 174 MHz), UHF-band (403 to 512 MHz), 800 MHz band (851 to 870 MHz), or 900 MHz band (935 to 941 MHz) are known as land mobile receivers, as these are the frequencies that the United States Federal Communications Commission (FCC) and/or other radio communications regulatory agencies of other countries have reserved for land mobile classified usage.
In general, such radios do not directly convert the radio frequency signal into an audible signal that constitutes a reproduction of the originally broadcast information signal. Instead, these radios usually include an IF section that first converts the RF signal to at least one IF signal (occasionally, a second or even a third IF stage will be used). This IF signal can then be processed to extract the information signal.
The specific frequency to which the IF section reduces (or, in some cases, increases) the RF signal cannot be selected at random. A poorly selected IF frequency can lead to poor receiver performance. For example, an IF frequency in the RF reception band poses obvious problems. Beyond this, harmonics of the various signals involved can restrict the list of potentially available frequencies for any given RF frequency even further. Therefore, although suitable IF frequencies for any given RF frequency signal of interest can be identified, care must be taken to avoid unsuitable IF frequencies
At the same time, radio receiver manufacturers have a strong incentive to try and standardize the IF frequencies of choice, as this greatly simplifies manufacturing and inventory control. For example, an IF section that provides satisfactory performance in radios that receive signals in either the low-band or mid-band land mobile spectrum would allow a manufacturer to build and stock only one IF section instead of two. In fact, such design criteria does prevail, and at least one manufacturer of land mobile radio receivers offers a selection of land mobile radio receivers that receive variously in the low-band, mid-band, high-band, UHF-band, and 800 MHz band that rely upon only two IF sections.
Unfortunately, to date, and despite a strongly perceived need in the industry, no one has identified a means of providing a single IF frequency that can adequately service reception in all of the above noted land mobile bands. The recent introduction by the FCC of additional land mobile spectrum in the 900 MHz band only complicated the problem further
Accordingly, there exists a need for a universal IF system suitable for use in land mobile radio receivers operating at any RF frequency currently allotted to this category of service. Further, radio performance should not be sacrificed to attain the efficiencies that would accompany use of such a universal system. In particular, EIA standard RS-204(c) defining high level reception in terms of spurious signal performance should be satisfied at all land mobile RF frequencies.